A wireless communications system typically includes a transmitter and a receiver for sending signals over carrier waves. The transmitter modulates information, such as voice, text, image, audio, and video data, onto a carrier before sending it to the receiver. The receiver demodulates the received signals to recover the original information. One of the typically used modulation schemes is IQ modulation, where I and Q represent in-phase and quadrature parts of a modulated signal. The IQ modulation is an efficient way of sending information, particularly in digital formats.
There are various factors that can cause the transmitter to generate imperfect modulated output signals. For example, in a quadrature modulation scheme, IQ imbalance, which occurs when there are different amplitude and phase distortions in I and Q channels, is one of the widely-recognized factors causing imperfect signals. The IQ imbalance induces sideband noise, which is essentially an image frequency interference “aliasing” into a desired signal band. The sideband noise degrades signal reception in a wireless communications system. The IQ imbalance has become a crucial issue as higher-level modulations appear in communications standards and low-cost, direct-conversion transmitters and receivers are increasingly employed in wireless communications systems.
Another typical factor causing imperfect signals is the hardware-induced carrier leakage. The carrier leakage, which is at the same frequency as the carrier, is often induced by the limited isolation between the carrier and output ports of the transmitter. On an IQ plane, a modulated signal with carrier leakage appears with a fixed offset vector.
Conventional solutions to combat carrier leakage rely on implementation of additional circuitry. The solutions for IQ imbalance are usually based on cancellation of the sideband noise in the transmitter, which also require additional circuitry or manufacturing calibration. For example, one can detect the sideband noise and carrier leakage by sampling an output signal, and minimize the same through hardware calibration. The signal sampling can be performed by using a testing receiver to detect output signals from a device under a test (DUT) transmitter.
One drawback of the conventional solutions is that the carrier leakage and sideband noise generated by the DUT transmitter cannot be distinguished from those generated by the testing receiver. This prevents an accurate calibration of the sideband noise and carrier leakage for the DUT transmitter.
As such, what is needed is a detection scheme to identify the sideband noise and carrier leakage of the DUT transmitter apart from those of the testing receiver.